Effect of sulfate attack on multiple interfacial transition zones of high-belite sulfoaluminate cement recycled concrete

Recycled aggregate concrete is a promising low carbon material, which need further analysis on the durability. To investigate the influence of sulfate erosion on recycled concrete made with high-belite sulfoaluminate cement (HBCSA), this study utilized compressive strength and microhardness test to evaluate the effect of sulfate on ITZ. Then by using backscattered electron imaging, scanning electron microscope and energy spectrometry, the mechanism for the micro- structural changes that occurred before and after sulfate erosion is revealed. Results showed that after 60 days of sulfate erosion, both aggregate-new mortar and new-old mortar interfaces display reduced microhardness and widened transition zones. Specifically, in the C30 group post- erosion, the microhardness values for the aggregate-new mortar interface ranged from 50.3 to 66.9 MPa, while the new-old mortar interface exhibited a range of 47-77 MPa. Besides, the new-old mortar interface exhibit marginally superior sulfate resistance to the aggregate-new mortar interface at the same concrete strength grade. Moreover, sulfate resistance improvs across all interfaces with increasing strength class. For instance, in the aggregate-new mortar interface, the maximum microhardness post-erosion for C40 showed a 51.6 % increase compared to C30. Through microstructure analysis, it is found that after sulfate erosion, the aggregate-new mortar and new-old mortar interfaces of HBCSA recycled concrete are both enriched with abundant ettringite and gypsum, which causes considerable swelling erosive product formation and even cracking. The less formed cracks in new-old mortar interfaces results in the better sulfate resistance. The finding is helpful for the future research on the high sulfate resistant recycled aggregate concrete.